The energy storage system refers to a system capable of storing extra electric power in a power system (grid) and using it at a necessary time point to efficiently use energy. In general, extra electric power is stored during a low load period such as nighttime, and the stored electric power is released during a heavy load period such as daytime. Therefore, electric power quality can be optimized through load leveling and the frequency control. It is anticipated that the ESS takes an important role in conversion of recently highlighted renewable energy sources into high quality electric power and interconnection of the converted electric power to a grid. In particular, the ESS is indispensable when a solar photovoltaic system or a wind turbine suffering from a serious output fluctuation is interconnected to the electrical grid.
The energy storage principle is simple. That is, electric energy received from a power system is ionized or converted into kinetic energy. The kinetic energy is stored as physically compressed and chemical energy. Then, the stored energy is converted and supplied to the power grid at a necessary timing. Various types of middle/large-sized energy storage systems are known in the art. For example, a lithium ion battery as a representative secondary battery has a separation membrane and electrolyte interposed between positive and negative electrodes, through which lithium ions are exchanged to store or discharge energy. It is known that the lithium ion battery exhibits excellent output characteristics and efficiency, but is still expensive. A sodium sulfur battery is operated by activating solid electrolyte capable of conducting sodium ions at a high temperature and is advantageously employed in a large capacity ESS. A redox flow battery is a type of rechargeable battery where energy is stored using an electrochemical reduction/oxidation potential and is suitable for long-time use in a large capacity. A supercapacitor having a fast response characteristic is employed in automotive regenerative braking systems and a short-term energy storage of a regenerative energy system for attenuating an output fluctuation. In addition, a flywheel and a compressed air storage device as a mechanical storage system are also highlighted as a mechanical energy storage ESS.